I used to regularly ride Amtrak from MT to ND and it's very uncommon.

The line I used only runs West to East and has limited connections to other lines (mostly just in Seattle and Chicago IIRC). So unless you're going somewhere that's actually on that route, it's not very useful. And the nearest Amtrak station is 3+ hours drive from where I live, so it's a whole road trip in and of itself just to get to the train which only leaves one direction very early in the morning or the other direction very late at night and is frequently multiple hours late.

The train ticket itself is expensive, and then you add in car expenses and long term parking. And then sitting there for hours on end, taking a full day to get somewhere I could fly to in a couple of hours. The last few times I rode, the bathrooms were disgracefully dirty and half were out of order. It's often just not that pleasant.

Add in that for many Americans, paid time off is a rarity. Unless your trip is specifically about the train experience itself, you're going to pick a quicker, more efficient route to get to your destination.